The present invention relates generally to a protection circuit for electrical apparatuses, and more particularly, to a method and apparatus for detecting when silicon controlled rectifier (SCR) control elements or other rectifying control elements in an engine driven generator of a welding-type power source have failed.
SCRs are known in the power electronics industry for their use as high power current switching devices. Such devices have found wide application in solid-state control systems where it is desired to control the voltage, current, and/or power supplied to a load. Generally, in single phase or poly-phase systems, one or more pairs of SCRs can be connected in a back-to-back relationship and in series with a load. The SCRs are further connected to an alternating current (AC) supply source, from which the current, voltage, or power desired to be controlled is provided. Typically, SCRs are triggered into conduction at an appropriate time in the positive and negative half cycles of the AC supply. The time between the zero crossing of the AC supply waveform and the moment at which the SCRs are driven into conduction is commonly referred to as the firing angle of the SCR. Adjusting the firing angle provides a means for controlling the voltage, current, and/or power delivered to the load.
Power sources for welding-type systems commonly use an SCR bridge, transistors, or diodes to control the welding-type output current using conventional phase control techniques. For static welding-type machines such as those that are powered by utility line supplies, shorted SCR detection circuits monitor the input voltage to the SCR bridge. If a short is present in the system the input voltage to the SCR bridge will go to zero. Because the short is applied through a failed device within an output rectifier bridge, the short is applied for only one-half of an AC cycle. Moreover, since machines powered by a utility line source will always have an input voltage to the SCR bridge under no-fault conditions, detection of a zero voltage at the input of the SCR bridge is indicative of a fault present in the system.
In contrast, engine driven generators will routinely have a zero voltage input to the SCR bridge during normal operation. For example, when the SCR bridge is turned on, the input voltage will be zero as a result of the high impedance of the generator. Further, the voltage may often collapse to zero when large currents are drawn from the generator. Because periods of zero voltages are not harmful and are expected in normal operation of an engine driven generator, detection circuits monitoring input voltage to the SCR bridge cannot discern between normal zero-going voltages or fault induced zero-going voltages. Therefore, electrical circuits that monitor input voltages are inapplicable with high impedance power sources such as engine driven generators.
Because voltage monitoring is not preferred for engine driven power sources, a number of systems have been developed that focus on input current to the SCR bridge to detect and determine shorted or fault conditions in the SCR. For example, fuses have commonly been used in each of the three-phase AC lines to detect high current flow. Another approach uses current transformers placed in each AC line to detect high current. While both of these approaches will indicate a shorted SCR bridge of an engine driven power source, these approaches require expensive and bulky external hardware that significantly increase the size, weight, and cost of the power source.
It would therefore be desirable to design an electrical circuit configured to detect shorted conditions of rectifying control elements in an engine driven generator of a welding-type power source.